Fix flaky test RepeatedSetup_KeepChangingPrivateDnsServers

Same as aosp/1262819, this test needs a buffer time to prevent being
flaky. The test running time increases by 700ms as a side effect.

Bug: 150678049
Bug: 152371971
Test: resolv_integration_test passed
Merged-In: I603e2a3b6977bcac06b542bc5312305fe25487fb
Change-Id: Ia6273ddcd039b83a18951a7d23fb6423f6e89c04
(cherry picked from commit 470bcc0e)